C++ Interview questions Part 9

C++ interview question: What is a node class?

Answer:A node class is a class that,

Ø relies on the base class for services and implementation,

Ø provides a wider interface to te users than its base class,

Ø relies primarily on virtual functions in its public interface

Ø depends on all its direct and indirect base class

Ø can be understood only in the context of the base class

Ø can be used as base for further derivation

Ø can be used to create objects.

A node class is a class that has added new services or functionality beyond the services inherited from its base class. 

C++ interview question: What is an orthogonal base class?

Answer:If two base classes have no overlapping methods or data they are said to be independent of, or orthogonal to each other. Orthogonal in the sense means that two classes operate in different dimensions and do not interfere with each other in any way. The same derived class may inherit such classes with no difficulty.

C++ interview question:What is a container class? What are the types of container classes?

Answer:A container class is a class that is used to hold objects in memory or external storage. A container class acts as a generic holder. A container class has a predefined behavior and a well-known interface. A container class is a supporting class whose purpose is to hide the topology used for maintaining the list of objects in memory. When a container class contains a group of mixed objects, the container is called a heterogeneous container; when the container is holding a group of objects that are all the same, the container is called a homogeneous container.

C++ interview question:What is a protocol class?

Answer:An abstract class is a protocol class if:

Ø it neither contains nor inherits from classes that contain member data, non-virtual functions, or private (or protected) members of any kind.

Ø it has a non-inline virtual destructor defined with an empty implementation,

Ø all member functions other than the destructor including inherited functions, are declared pure virtual functions and left undefined.

C++ interview question:What is a mixin class?

Answer:A class that provides some but not all of the implementation for a virtual base class is often called mixin. Derivation done just for the purpose of redefining the virtual functions in the base classes is often called mixin inheritance. Mixin classes typically don't share common bases.

C++ interview question:What is a concrete class?

Answer:A concrete class is used to define a useful object that can be instantiated as an automatic variable on the program stack. The implementation of a concrete class is defined. The concrete class is not intended to be a base class and no attempt to minimize dependency on other classes in the implementation or behavior of the class.

C++ interview question:What is the handle class?

Answer:A handle is a class that maintains a pointer to an object that is programmatically accessible through the public interface of the handle class.

Explanation:

           In case of abstract classes, unless one manipulates the objects of these classes through pointers and references, the benefits of the virtual functions are lost. User code may become dependent on details of implementation classes because an abstract type cannot be allocated statistically or on the stack without its size being known. Using pointers or references implies that the burden of memory management falls on the user. Another limitation of abstract class object is of fixed size. Classes however are used to represent concepts that require varying amounts of storage to implement them.

           A popular technique for dealing with these issues is to separate what is used as a single object in two parts: a handle providing the user interface and a representation holding all or most of the object's state. The connection between the handle and the representation is typically a pointer in the handle. Often, handles have a bit more data than the simple representation pointer, but not much more. Hence the layout of the handle is typically stable, even when the representation changes and also that handles are small enough to move around relatively freely so that the user needn’t use the pointers and the references. 

C++ interview question:What is an action class?

Answer:The simplest and most obvious way to specify an action in C++ is to write a function. However, if the action has to be delayed, has to be transmitted 'elsewhere' before being performed, requires its own data, has to be combined with other actions, etc then it often becomes attractive to provide the action in the form of a class that can execute the desired action and provide other services as well. Manipulators used with iostreams is an obvious example.

Explanation:

A common form of action class is a simple class containing just one virtual function.

class Action

{

public:

virtual int do_it( int )=0;

virtual ~Action( );

}

Given this, we can write code say a member that can store actions for later execution without using pointers to functions, without knowing anything about the objects involved, and without even knowing the name of the operation it invokes. For example:

class write_file : public Action

{

File& f;

public:

int do_it(int)

{

return fwrite( ).suceed( );

}

};

class error_message: public Action

{

response_box db(message.cstr( ),"Continue","Cancel","Retry");

switch (db.getresponse( ))

{

case 0: return 0;

case 1: abort();

case 2: current_operation.redo( );return 1;

}

}; 

A user of the Action class will be completely isolated from any knowledge of derived classes such as write_file and error_message.